The selection of any circuit breaker, for
any given duty, is ultimately based on an assessment of its ability to perform
the following basic functions:

a) To carry the required full-load current
without overheating (i.e., it should have the correct current rating),

b) To switch and isolate or disconnect the
load from the source at the given system voltage (i.e., it should have the
correct voltage rating),

c) To interrupt any possible abnormally
high operating current or short-circuit current likely to be encountered during
operation (i.e., it should have the correct interrupting rating), and

d) To be able to perform these functions
over an acceptably long period of time under the operating and environmental
conditions that will actually prevail in the application (i.e., it should have
the correct mounting provisions, enclosure, and accessories for operation in
the environment in which it is to be applied).

The degree to which a circuit breaker can
satisfy these requirements is a measure of its applicability for a function. A
circuit breaker's rating indicates these capabilities to the user because
rating is established by proof testing. Hence, an understanding of how a
circuit breaker of any given type is tested will give insight to its
applicability for any function.

The role of standards

The primary vehicle for ensuring
commonality in performance among circuit breakers of the same rating produced
by different manufacturers is a product standard. Standards represent the
consensus of manufacturers about what a given product should be able to do as a
minimum.

Standards establish the design tests that
each manufacturer must perform and pass in order to claim a rating and to be in
compliance with that standard. Some standards include requirements for periodic
follow-up testing which, in effect, continues to sample the capabilities of
newly manufactured circuit breakers.

This assures that they maintain the
capabilities of their product ratings. Standards provisions also provide for
monitoring the quality of the materials used in the construction of circuit
breakers and the quality of the workmanship in the manufacturing process.

As stated previously, standards
requirements for the different classes of circuit breakers establish a basis
for minimum performance. Circuit breakers may prove by test to perform better
than their product ratings indicate, but they can never be permitted to perform
worse.

The
user, however, may never assume that a circuit breaker can perform better than
its rating indicates and should realize that there are manufacturing variations
among mass-produced products. The levels of performance required by the
standards for the minimum acceptable performance of different classes of
circuit breakers will be the primary references in the discussion that follows.

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